Problems after updating to Microsoft Windows 10

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The warning you're seeing is telling you that you haven't assigned a default program to open this type of file (.err). To fix:

1. Navigate to the directory that contains the eplusou.err file in windows explorer
2. Right click the file > Open with > choose default program > select a text editor like Notepad, Notepad++, etc.
3. Close OpenStudio and reopen
4. Next time you click the eplusout.err file in OpenStudio, it should open with the text editor you chose

This should work for any file types with an extension. I have never figured out how to assign a default program to files with no extension (like stderr or stdout files) in Windows.